Scientists Just Discovered a Missing Link Between San Francisco’s Faults

Two of California’s most active fault lines appear to be a 118-mile-long fault instead

San Francisco

If you grew up in San Francisco, earthquake drills were just as common as fire drills. This fear of “the big one” is part of California life—especially given the large number of fault zones that underlie the San Francisco Bay Area. And it turns out that the area’s earthquake risk is even higher than once thought: As William Herkewitz reports for Popular Mechanics, scientists have now discovered that two of San Francisco’s most notorious faults are actually one.

In a new study published in the journal Science Advances, geologists present what they call a “missing link” between the Hayward and Rodgers Creek faults. The Hayward Fault runs along the East Bay hills, and the Rodgers Creek cuts up toward Napa and Sonoma. The two faults have long been considered the most likely to cause the area’s next major quake, with a 31 percent probability it will rupture and create a magnitude 6.7 or larger earthquake within the next 30 years. But they have always been considered distinct by geologists, who though of them as parallel or separated by a large gap inside the San Pablo Bay.

Not so: The new study used a technology called seismic reflection to inspect the hard-to-characterize seafloor beneath the bay. They bounced sound waves off of the mud that lies atop the ocean floor—a substance that confounded previous researchers because it muddies the ways in which sound waves are absorbed. By studying the mud itself and combining those measurements with magnetic information about the rock below, they were able to spot a never-before-seen connection between the two faults.

“That’s a big deal,” David Ponce of the USGS tells Herkewitz—and he’s not exaggerating. The newly understood, longer fault is essentially double the size of either short fault. Not only does it stretch beneath an extremely populous region, but because it’s connected, shaking on one or the other side of the fault will likely travel straight through the entire thing.

In their paper, the researchers state that the earthquake produced by the larger fault could be up to a 7.4 in magnitude. That’s more than five times stronger than the 1989 Loma Prieta quake, which was a 6.9 magnitude. That quake killed 69 people and injured thousands, damaging thousands of homes and businesses.

The paper coincided with California’s Great ShakeOut Drill, a statewide drill aimed at helping the public practice and prepare for what to do when a major earthquake strikes. As John Gregory reports for ABC 7, an estimated 10.6 million people took part in the drill.

As always, the message is clear: When it comes to earthquakes in San Francisco, it’s not a question of if, but when. New discoveries about the power and magnitude of California’s fault system may be sobering—but the more people know about what’s going on beneath the surface, the more prepared they can be for the unpredictable and inevitable.

Get the latest stories in your inbox every weekday.